The roaring dell, o'erwooded, narrow, deep, And only speckled by the mid-day sun; Where its slim trunk the ash from rock to rock Flings arching like a bridge; -- that branchless ash, Unsunn'd and damp, whose few poor yellow leaves Ne'er tremble in the gale, yet tremble still, Fann'd by the water-fall!
His friends enter the dense valley or ravine.
The valley is your typical "deep, dark forest," crowded with trees, moss, and gnarled branches. You can barely see a few splashes of sunshine here and there at midday. It must be particularly hard to see now that it is close to evening (see the epigraph). The sunlight must travel through the dense foliage, giving it a "speckled" appearance.
The speaker remembers a fallen "ash" tree that spans from "rock to rock" across the stream. Maybe his friends will have to cross this natural "bridge" to get to the other side.
The ash receives no sunlight and does not move or "tremble" in the wind, because there is no wind this far down in the valley. Instead, it "trembles" from the movement of the air created by the water fall, like being "fann'd."
and there my friends Behold the dark green file of long lank weeds, That all at once (a most fantastic sight!) Still nod and drip beneath the dripping edge Of the blue clay-stone.
His friends see the long, dark, green weeds that hang over the edge of the rock, the "blue clay-stone," next to the waterfall. The weeds drip from the spray of the falls.
The speaker thinks these weeds are particularly awesome ("fantastic"). They thrive even in seemingly harsh conditions.
If we wanted to be critical, we'd say these weeds are even hardier than our speaker: if they had legs, they wouldn't let no scalding milk prevent them from talking a walk!